The present invention relates to an advantageous process for the hydrogenation of phenols to give the corresponding cyclohexanones using palladium-on-carbon catalysts.
Processes for the hydrogenation of phenols over palladium-on-carbon catalysts to give cyclohexanones are known in principle (see, for example, DE-A 2 909 780, DE-A 2 530 759 and U.S. Pat. No. 2,829,166).
In general, to achieve a good selectivity, a hydrogen pressure of less than 20 bar and a reaction temperature in the range from 120 to 250.degree. C. are sought and an alkaline compound is added to the hydrogenation mixture. The catalyst can, if desired, be treated with alkali metal ions such as Na or K ions (see, for example, German Auslegeschrift 1 144 267 (U.S. Pat. No. 3,076,810)). The use of phenoxides and bicarbonates is described in FR-A 2 372 136 (G.B. 1,563,044)).
According to DE-A 2 909 780, sodium carbonate, borax and/or sodium acetate are used for the hydrogenation of p-tert-amylphenol. At the same time, it is indicated here that the use of palladium catalysts having metal surface areas of 15 m.sup.2 /g and more is promising. Preference is given to using palladium-on-carbon catalysts containing 5% by weight of palladium, based on a support which has a surface area of 800 m.sup.2 /g.
When using these known processes, in particular when they are extended to further substituted phenols, it is found that the hydrogenation results (e.g. yield of desired cyclohexanones) and the hydrogenation times required fluctuate greatly in repeat batches under the same conditions (see Examples 14 to 19).
Since, in many cases, cyclohexanones are used as starting compounds for pharmaceuticals and crop protection agents, a high purity has to be ensured. Especially in the case of difficult-to-remove secondary components, the value of a preparative process therefore depends on the achievement of a very high selectivity together with good reproducibility of the results. Furthermore, greatly varying hydrogenation results and times result in problems in carrying out the reaction and in the work-up of the reaction mixtures.